1. Field of the Invention
The present invention relates to a method of manufacturing a nitride semiconductor light emitting element and to a nitride semiconductor light emitting element, and more particularly to a method of manufacturing a nitride semiconductor light emitting element and to a nitride semiconductor light emitting element capable of use as a vertical cavity surface emitting laser (VCSEL), a light emitting diode (LED), a photodetector (PD) or a combination of these devices.
2. Background Information
Studies on laser elements capable of functioning as a vertical cavity surface emitting laser have been conducted using a nitride semiconductor. However, in fact, there is only one reference in the world reporting oscillation of a vertical cavity surface emitting laser of nitride semiconductor by current injection (Applied physics letters, vol. 92, p 141102). Such a vertical cavity surface emitting laser has, as shown in FIG. 14, a construction in which a Bragg reflector 51, an n-type nitride semiconductor layer 52, a light emitting layer 53, and a p-type nitride semiconductor layer 54 are stacked on a sapphire substrate 50 in this order. The Bragg reflector 51 is made of a multilayer film of AlN/GaN.
Such a vertical cavity surface emitting laser has an insulating film 55 made of SiN formed on a side surface of a nitride semiconductor layer and an upper surface of a p-type nitride semiconductor layer 54, a transparent electrode 57 formed on the p-type nitride semiconductor layer 54, and a p-electrode 56 formed above the insulating film 55 so as to contact with the transparent electrode 57 and has an aperture. Formed over the transparent electrode 57 which is an aperture of the p-electrode 56 is a Bragg reflector 58 made of a dielectric multilayer film.
Further, a part of an n-type nitride semiconductor layer 52, the light emitting layer 53 and the p-type nitride semiconductor layer 54 are etched from the p-type nitride semiconductor layer 54 to form a protrusion shape with the n-type nitride semiconductor layer 52 exposed at the bottom, and an n-electrode 59 is formed on the exposed n-type nitride semiconductor layer 52.
Also, proposed is a method of fabricating a vertical cavity surface emitting laser, in which a nitride semiconductor layer and a first distributed Bragg reflector are formed on a sapphire substrate, an other substrate is bonded on the first distributed Bragg reflector, the sapphire substrate is removed, a second distributed Bragg reflector is formed so as to face the first distributed Bragg reflector, and electrodes are formed on the surfaces of the semiconductor layer (JP 2000-228562A, JP 2000-228563A, and JP 2003-234542A). Further, proposed is a surface emitting semiconductor laser in which periphery of the element is etched (JP H05-190979A). Moreover, proposed is a nitride semiconductor light emitting element in which after forming a mesa in a semiconductor layer, a part of the layer is oxidized (JP 2006-216816A).
In a case where a Bragg reflector is made of a nitride semiconductor, a Bragg reflector having electric conductivity and a high reflectance is difficult to obtain. As a result, a Bragg reflector made of a nitride semiconductor with extremely low electrical conductivity as described above or a dielectric material is usually used. Therefore, in a vertical cavity surface emitting laser, it is difficult to efficiently supply electric current to a light emitting layer which is interposed between the Brag reflectors facing each other. Also, as in a conventional structure, when a reflector is made of an insulating nitride semiconductor with extremely low electrical conductivity or a dielectric material, and two electric contact portions are formed on an upper surface (on the same plane) of the p-type layer and the n-type layer, uniform injection of electric current into the region contributing to laser oscillation (hereinafter referred to as the “element region”) is difficult to achieve and the current density becomes locally high. Therefore, the matching of lateral distribution of light and gain becomes insufficient, resulting in a high threshold current. In addition, the current input becomes high, so that heat generated in the element region cannot be released, resulting in significant reduction of the emission efficiency.
On the other hand, as described in JP 2000-228562A, JP 2000-228563A, and JP 2003-234542A, in a case where using an other substrate which is different from a growth substrate, a p-electrode is formed in a region away from the element region, the electric current is difficult to spread in the p-type layer in a nitride semiconductor. Therefore, the electric current tends to crowd in a peripheral portion of the p-electrode and the current density becomes locally high. As a result, the electric current cannot be injected into the element region, so that emission and oscillation are difficult to achieve.
In addition, in a case where a mesa structure or a selectively oxidized layer is formed to reduce the threshold, the electric current path is narrowed and the heat generation is significantly increased.